# std::binary_function

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
template< class Arg1, class Arg2, class Result > struct binary_function; |  | (deprecated in C++11) (removed in C++17)

`std::binary_function` is a base class for creating function objects with two arguments.

`std::binary_function` does not define operator(); it is expected that derived classes will define this. `std::binary_function` provides only three types - `first_argument_type`, `second_argument_type` and `result_type` - defined by the template parameters.

Some standard library function object adaptors, such as std::not2, require the function objects they adapt to have certain types defined; std::not2 requires the function object being adapted to have two types named `first_argument_type` and `second_argument_type`. Deriving function objects that take two arguments from `std::binary_function` is an easy way to make them compatible with those adaptors.

`std::binary_function` is deprecated in C++11 and removed in C++17.

### Member types

Type | Definition
`first_argument_type` | `Arg1`
`second_argument_type` | `Arg2`
`result_type` | `Result`

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <vector>

    struct same : std::binary_function<int, int, bool>
    {
        bool operator()(int a, int b) const { return a == b; }
    };

    int main()
    {
        std::vector<char> v1{'A', 'B', 'C', 'D', 'E'};
        std::vector<char> v2{'E', 'D', 'C', 'B', 'A'};
        std::vector<bool> v3(v1.size());

        std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(), std::not2(same()));

        std::cout << std::boolalpha;
        for (std::size_t i = 0; i < v1.size(); ++i)
            std::cout << v1[i] << " != " << v2[i] << " : " << v3[i] << '\n';
    }

Output:

    A != E : true
    B != D : true
    C != C : false
    D != B : true
    E != A : true

### See also

function (C++11) | copyable wrapper of any copy constructible callable object (class template)
move_only_function (C++23) | move-only wrapper of any callable object that supports qualifiers in a given call signature (class template)
ptr_fun (deprecated in C++11)(removed in C++17) | creates an adaptor-compatible function object wrapper from a pointer to function (function template)
pointer_to_binary_function (deprecated in C++11)(removed in C++17) | adaptor-compatible wrapper for a pointer to binary function (class template)
unary_function (deprecated in C++11)(removed in C++17) | adaptor-compatible unary function base class (class template)

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